1. Field of the Invention
The present invention relates to a servo control apparatus which controls a controlled system by a feedback control, and more particularly, to a servo control apparatus which estimates a disturbance applied to the controlled system, and carries out a feedback control while compensating an actual disturbance on the basis of the estimated disturbance.
2. Description of the Related Art
In recent years, the following research and development have been actively made. When controlling a controlled system by a feedback control, a disturbance applied to the controlled system is estimated, and a manipulated variable considering the estimated disturbance is applied to the controlled system, and thereby, a feedback control is carried out while compensating an actual disturbance on the basis of the estimated disturbance.
Thus, a state observer has recently attracted interest as a preferable device for estimating the disturbance.
The principle of the state observer will be described below.
The observer is a device for estimating a state incapable of being actually detected, from a detectable state. The state incapable of being actually detected is a state where a disturbance is actually applied to the controlled system, for example. The observer estimates a disturbance applied to the controlled system, and then, computes a compensated variable capable of canceling the actual disturbance, and further, adds the compensated variable to a feedback control object so as to compensate the actual disturbance.
Next, a disturbance estimating process by the observer will be described below with reference to FIG. 13. FIG. 13 shows the case of applying the observer with respect to a focus servo control object which controls a focus actuator included in an optical disk reproducing apparatus such as a CD (Compact Disk) player as a controlled system. In particular, FIG. 13 shows a feedback servo loop formed in the focus servo control system.
The focus actuator (hereinafter, referred simply to as actuator) is an actuator for moving an objective lens in the direction perpendicular to an information recording surface of an optical disk, to focus a light beam on the information recording surface of the optical disk.
In FIG. 13, a controlled object U(s) is set as an actuator, and a controlled variable y is set as a position of the actuator in the direction perpendicular to the optical disk.
Now, a transfer function of the actuator is expressed as a second-order lag control system as follows. EQU U(s)=A.times.wa.sup.2 /(S.sup.2 +2.times.ka.times.wa.times.s+wa.sup.2) (1)
where, A is a gain (m/Ampere) of actuator, ka is a viscous braking coefficient of the actuator, and wa is a natural vibration frequency (rad/sec) of the actuator.
Next, supposing a conversion sensitivity for a focus error signal output in the actuator as a positional detection sensitivity Ke (Volt/m), the following equation (2) is formed. Incidentally, the conversion sensitivity is determined by a sensitivity of photo-detector and an amplification factor of an error generating amplifier included in the optical disk reproducing apparatus. EQU REF-y.times.Ke=er (2)
where, REF is a desired value on which an actuator should be positioned, er is an error in the aforementioned feedback control system. As shown in FIG. 13, the error er obtained by the above equation (2) is supplied to one input terminal of an observer.
In FIG. 13, a relationship between a manipulated variable (voltage value) u and a drive current i for driving the actuator is expressed as follows. EQU i=Kdr.times.u (3)
where, Kdr (Ampere/Volt) is a voltage/current conversion sensitivity of a driver. The driver is controlled by the manipulated variable u, thereby generating the drive current i. The drive current i is converted into a drive voltage v by a current/voltage converter having a current/voltage conversion sensitivity Kiv (Volt/Ampere) as shown in the following equation (4), and then, is supplied to the other input terminal of the observer. EQU V=Kiv.times.i (4)
where, the current/voltage conversion Kiv is equivalent to a conversion sensitivity with respect to the feed back of the drive current i to the observer. Namely, the current/voltage conversion Kiv corresponds to a so-called return resistance.
Next, in order to simplify an explanation, a disturbance applied to an actuator is regarded as only disturbance with respect to a certain position. Then, as shown in FIG. 13, if a disturbance variable is set as d, the following equation (5) is formed. EQU i.times.U(s)+d=y (5)
Here, in the above equation (2), when the desired value REF is set as zero (REF=0), the equation (2) becomes the following equation. EQU y.times.Ke=-er (2A)
Further, the following equation is obtained from the above equation (4). EQU i=v/Kiv (4A)
Further, if i and y in the equation (5) are eliminated by using the equations (2A) and (4A), the following equation is formed. EQU (v/Kiv).times.U(s)+d=-er/Ke
When arranging the this equation, the disturbance variable d is expressed as shown in the following equation (6), using the input voltage v and the error er supplied to the observer. EQU d=-er/Ke-(v/Kiv).times.U(s) (6)
In this case, parameters schematically showing an interior of the observer is expressed as nominal values, and an additional character n is appended to each nominal value in expression in order to distinguish actual control elements from the nominal value. Namely, the positional detection sensitivity Ke is expressed as a positional detection sensitivity nominal value Ken, the voltage/current conversion sensitivity Kdr is expressed as a voltage/current conversion sensitivity nominal value Kdrn, the current/voltage conversion sensitivity Kiv is expressed as a current/voltage conversion sensitivity nominal value Kivn, and the controlled system U(s) is expressed as a controlled system nominal value Un(s).
Incidentally, the nominal value is a torque rated value of a spindle motor for rotating the optical disk in the optical disk reproducing apparatus, and is indicative of a value shown by a performance indication of the optical disk reproducing apparatus. If there is no performance indication, the nominal value is determined by an experiment or the like, or is computed (calculated) from a theoretical calculation. Incidentally, there is the case where the nominal value and an actual control element are not always equal to each other due to factors such as inaccurate determination or calculation, aged deterioration, temperature change or the like.
On the basis of the above equation (6), when an estimated disturbance variable DOBS, which is an estimated variable of the disturbance d, is expressed using each nominal value, the following equation (7) is obtained. EQU DOBS=-er/Ken-(v/Kivn).times.Un(s) (7)
Thus, as is evident from the above description, it is possible to estimate and compute the estimated disturbance variable DOBS from the input voltage v and the error er by using the observer without detecting the actual disturbance d.
Moreover, in FIG. 13, the estimated disturbance variable DOBS is converted into a compensated variable h by a robust filter R(s), and then, the compensated variable h is added to a variable which is obtained by phase-compensating the error er with a phase compensator C(s), and thus, a manipulated variable u is generated so as to suppress the disturbance d.
On the other hand, in recent years, it is general that the total process of feedback control system including the observer is digitally carried out by one DSP (Digital Signal Processor) at a high speed and high accuracy.
However, it is general that the above controlled system U(s) is driven by a drive current i which is usually an analog signal. Thus, it is a frequent case that a desired value REF, which is a target of control, is also an analog signal.
Therefore, in the case of treating these analog signals with the DSP which is a digital circuit, in FIG. 13, at least an A/D converter for digitizing the error er, an A/D converter for digitizing the drive voltage v, and a D/A converter for analogizing the manipulated variable u are needed. As a result, the structure of the servo control device becomes complicate. Further, if the servo control device is realized as an IC chip, an occupied area increases, and a production cost becomes high.
Moreover, the above description has shown the case where the controlled system U(s) is a focus actuator. In the case where the controlled system U(s) is a tracking actuator, a slider motor or a spindle motor, the problems similar to above take place. Incidentally, the tracking actuator is a device for finely adjusting a light beam irradiation position in the radial direction of the optical disk by moving the objective lens in the direction parallel to the information recording surface of the optical disk. The slider motor is a device for roughly adjusting a radial position of light beam irradiation position by moving a pickup. The spindle motor is a device for controlling a rotational frequency of the optical disk.